ABSTRACT: Fifty-eight years ago, Harberger (1954) estimated that the costs of monopoly, which resulted from misallocation of resources across industries, were trivial. Others showed the same was true for tariffs. This research soon led to the consensus that monopoly costs are of little significance—a consensus that persists to this day. This paper reports on a new literature that takes a different approach to the costs of monopoly. It examines the costs of monopoly and tariffs within industries. In particular, it examines the histories of industries in which a monopoly is destroyed (or tariffs greatly reduced) and the industry transitions quickly from monopoly to competition. If there are costs to monopoly and high tariffs within industries, we should be able to see these costs whittled away as the monopoly is destroyed. In contrast to the prevailing consensus, this new research has identified significant costs of monopoly. Monopoly (and high tariffs) is shown to significantly lower productivity within establishments. It also leads to misallocation within industry: resources are transferred from high to low productivity establishments. From these histories a common theme (or theory) emerges as to why monopoly is costly. When a monopoly is created, “rents” are created. Conflict emerges among shareholders, managers, and employees of the monopoly as they negotiate how to divide these rents. Mechanisms are set up to split the rents. These mechanisms are often means to reduce competition among members of the monopoly. Although the mechanisms divide rents, they also destroy them (by leading to low productivity and misallocation).